A common arrangement for the energy stabilization of accelerated charged particle beams employs momentum analysis of a collimated beam followed by a monitoring arrangement in which current sensors are disposed proximate to the beam, peripheral to the main portion thereof, to sample the analyzed beam width (in the plane of the momentum analysis). A variation in the difference between these current sensors comprises an error signal which is employed to actuate a servo system for correction appropriate to the type of accelerator. Such an arrangement requires a massive momentum analyzer and constrains the geometry of the entire system. Moreover, the current sensors typically intercept a portion of the analyzed beam and becomes sources of secondary radiation. Another arrangement, particularly common in radiation therapy and industrial radiation systems of prior art utilizing momentum analysis (magnets) comprises momentum analysis within a bend magnet, which is typically achromatic, in which no signals are derived from analyzer slits, but where sensors are placed within the radiation field downstream of the magnet. These sensors, typically located within a transmission ion chamber, will detect average and differential intensity across the radiation field. The magnet setting determines the mean energy: the sensors with appropriate servos, maintain intensity and symmetry. Where such systems do not employ a momentum analysis, the error signals derived from the ionization chamber merely maintain geometric stability or output of the charged particle beam, while true energy stability is not maintained. It is also known for prior art radiation therapy equipment to utilize an error signal derived in the above manner from a momentum analyzed beam to affect the energy of the unanalyzed beam by adjustment of some operating parameter of the accelerator.
While it is known in prior art to employ transmission ion chambers to obtain a signal proportional to angular intensity of the beam, referenced to a desired beam axis, the utility of this error signal has been employed in prior art to correct purely geometric properties of the beam or, in the alternative to cooperate with a momentum analyzer for energy stabilization.
In the prior art, it is also known to monitor the symmetry properties of an X-ray beam at a point downstream of a flattening filter and to associate detected asymmetry of a photon flux with an energy excursion of the primary electron beam. This association depends upon the angular intensity distribution of the X-ray production and is sensitivity to energy variations downstream from a flattening filter. An example of this prior art which is restricted to X-ray beams and the use of a target and flattening filter is described in U.S. Pat. No. 4,347,547.
In the present invention, an unanalyzed beam of small cross section is incident on a thin scattering foil and a measure of the angular distribution of the scattered flux is obtained. For an incident beam I.sub.0, energy E, the scattered flux at scattering angle .theta. is given by EQU I(.theta.)=I.sub.0 e.sup.-kE
where k depends upon the properties of the scatterer. If the scattered flux is monitored for two discrete angular intervals, .theta..sub.1 of width .DELTA..theta..sub.1 and .theta..sub.2 of width .DELTA..theta..sub.2, then, ignoring the angular widths ps EQU I(.theta..sub.1)/I(.theta..sub.2)=e.sup.-k(E1-E2) (Equ. 1)
where E1=E(.theta..sub.1) and E2=E(.theta..sub.2). Thus, the energy distribution in the unanalyzed beam is functionally related to the angular distribution. A particularly simple arrangement is achieved for choice of one of the angles .theta.=0.degree.
The angular data is preferentially derived from transmission ion chamber signals to minimize secondary radiation. These signals are proportional to the flux scattered into the path traversing respective ion chamber electrodes. These latter may form a composite (for example) coplanar arrangement, or alternatively multiple ion chambers may be disposed along the axis.
A radiation therapy machine typically comprises a microwave electron accelerator mounted on a gantry. In the prior art, if an energy analysis magnet is used it deflects the accelerator beam through 90.degree. or 270.degree., and the analyzed beam is derived along an axis directed toward an isocenter about which the gantry rotates. The gantry preferably provides two degrees of rotational freedom to permit the beam to be incident on the isocenter from a variety of directions.
A simular arrangement may also be utilized in accelerators for industrial applications, although in this case there is generally no fixed isocenter.
The requirement for an analyzing magnet adds considerable mass to the system and interposes an extension to path length which enlarges the required clearances for rotation of the equipment. However, a simple linear accelerator, without means for analysis of the kinetic properties of the beam, requires other means for detection and prompt correction of energy instability in the accelerated beam.